You can also check the file /var/run/mongodb/mongod.pid for a valid process ID value. Sometimes it might be necessary to create the parent folder so that the PID can be created:

$ sudo mkdir /var/run/mongodb/

You could also check the mongoDB log for other errors:

$ tail /var/log/mongodb/mongod.log

If the mongod service is not active, double-check that folders are named correctly in configuration and service files, and that permissions and ownership are set correctly on those folders. If anything is not named and attributed correctly, then the service will likely not start and note something like the following error: